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Rain gardens are vegetated surface depressions, often located at low points in landscapes, designed to receive stormwater runoff from roads, roofs, and parking lots. The gardens’ sandy soils allow stormwater to drain quickly to the native soils below and eventually to groundwater. The rain garden vegetation and soils remove pollutants and nutrients from stormwater runoff through biological and physical processes such as plant uptake and sorption to soil particles. In comparison with stormwater release to receiving waters through conventional storm drain systems, infiltrating stormwater through rain gardens reduces peak flows and loadings of both pollutants and nutrients. This reduction improves the physical and biological integrity of receiving streams by reducing stream bank erosion and negative effects on stream communities.
While local governments and individual homeowners are building these systems, relatively few scientific studies have documented the ability of rain gardens to remove pollutants and nutrients. This U.S. EPA long-term research project investigates: 1) the performance of rain gardens in removing pollutants, and 2) whether currently-accepted design standards can be adjusted to improve nitrate removal capabilities.
Typical rain garden designs provide large removals of pollutants of concern, including heavy metals, phosphorus, total nitrogen, and ammonium. The gardens have been less successful in removing nitrate, an important contributor to downstream water quality degradation. A possible explanation for this pattern is that the currently-accepted rain garden design inhibits microbial processes that convert nitrate to nitrogen gas, which is released to the atmosphere, thus keeping the nitrate out of the groundwater and receiving streams. These microbial organisms require wet conditions and a readily-available source of carbon to drive the reaction. Rain gardens’ sandy soils drain quickly and have low levels of available carbon, creating poor conditions for these microbes.
U.S. EPA’s research is investigating various soil carbon amendments and wet conditions in deep soils in rain gardens to increase nitrate removal. Experiments include adding unprinted newspaper to the soil as a carbon source and a wet zone to deep soils. Nitrate concentrations in water draining from these rain gardens will be compared t